1. Field of the Invention
This invention relates to a moving vector detecting device which is used for highly efficient coding of a moving picture and, more particularly, to the development of a moving vector with half-picture-element pitch accuracy.
2. Description of the Prior Art
For highly efficient coding of a moving picture, a system for performing DCT conversion (Discrete Cosine Transform) of a difference of picture element data between frames and for variable-length-coding of an output of the DCT conversion has been proposed. To efficiently compress the moving picture, a moving vector detector is provided, and a detected moving vector is transmitted. Also, to develop an interframe difference, a local decoder for developing a prediction value of a prevision frame is provided. The detected moving vector is fed to this local decoder.
An example a method for moving vector detection is block matching. FIG. 1A shows a block BL0 formed of many (3.times.3) blocks obtained by the division of a picture of a previous frame. Picture element data of B11, B12, . . . , B33 is contained in the block BL0. FIG. 1B shows a block AL0 which includes picture element data A11, A12, . . . , A33 in a picture of the present frame and from which a moving vector is to be obtained. Within a range of search for the moving vector, the position of the block AL0 of the present frame is predicted from the previous frame.
First, an absolute value (ABS) (called "residual difference"), which is a difference of value of picture elements of corresponding positions between two frames in terms of time, is calculated. Then residual difference for each element is added for every block and a total sum .DELTA.F of the residual differences is obtained. EQU .DELTA.F=ABS{(A11-B11)+(A12-B12)+. . . +(A33-B33)}
Plural total sums .DELTA.F of the residual differences are obtained in the range of search of a moving vector. A minimum value of the plural .DELTA.F is detected, and a position (x, y) of the minimum value is detected as the moving vector. The moving vector (x, y) is the resolving power of the distance (one-picture-element pitch) between picture elements.
In the above-described highly efficient coding method, the higher the accuracy of the moving vector, the lower the distortion of coding. As a result, it has been proposed that the moving vector be detected with the accuracy of a half pitch of one picture element (called "half-picture-element pitch"). This means that a prediction block is formed by calculating the average value of two or four points and block matching processing is done with respect to the prediction block.
More specifically, assuming that a corresponding block in a previous frame, which is obtained with the one-picture-element pitch, is BL0, the following prediction blocks, which are shifted by the half-picture-element pitch, are obtained by the calculation of an average between BL0 and a block shifted by the one-picture-element with respect to BL0:
prediction block BL1 of (x+0.5, y); PA1 prediction block BL2 of {(B11 to B33)+(B12 to B34)}/2 (x-0.5, y); PA1 prediction block BL3 of {(B11 to B33)+(B10 to B32)}/2 (x+0.5, y+0.5); PA1 prediction block BL4 of {(B11 to B33)+(B10 to B32)+(B20 to B42)+(B21 to B43)}/4 (x-0.5, y+0.5); PA1 prediction block BL5 of {(B11 to B33)+(B12 to B34)+(B01 to B23)+(B02 to B24)}/4 (x+0.5, y-0.5); PA1 prediction block BL6 of {(B11 to B33)+(B10 to B32)+(B01 to B23)+(B00 to B22)}/4 (x-0.5, y-0.5); PA1 prediction block BL7 of {(B11 to B33)+(B21 to B43)}/2 (x, y+0.5); PA1 prediction block BL8 of {(B11 to B33)+(B01 to B23)}/2 (x, y-0.5);
By detecting a prediction block Bi whose sum of absolute values of differences between a present frame block AL0 and a prediction block BLi (i=0, 1, 2, . . . , 8) takes a minimum value, a vector corresponding to this detected prediction block Bi will have a half-picture-element-pitch accuracy. By examining the calculation amount by a method for obtaining such a moving vector of the half-picture-element pitch, the following is understood.
Now, assuming that the size of one block is (N.times.N), the amount of calculations after obtaining the moving vector of the one-picture-element pitch is examined. First, the calculations of prediction blocks are limited to only additions. As a result, EQU (Additions for prediction of two-point average)+(Additions for prediction of four-point average)=4N.sup.2 +6.times.2N.sup.2 =16N.sup.2
To obtain an absolute value of a difference, the subtractions of 4N.sup.2 and the generation of the absolute values of 4N.sup.2 are needed for each prediction of each two-point average and the prediction of the four-point average. Therefore, EQU (Subtractions)+(Absolute value generation)=(8N.sup.2)+(8N.sup.2)=16N.sup.2.